This invention relates to composite materials containing cellulosic pulp fibers dispersed in a polymeric matrix. The invention also relates to methods by which to produce such composites and molded articles therefrom.
Composites are widely used in a broad spectrum of applications, including automotive parts, sporting goods, computer chips, and the like. Composites are generally defined as a macroscale combination of two or more solid components that are insoluble in each other and which further differ in chemical nature. More particularly, composites typically include at least one reinforcing component enveloped in a matrix composition. The reinforcement generally bears the load to which the composite is subjected, while the matrix transfers the load between the reinforcing elements. An interface is formed between the reinforcement and the matrix. It is the adhesion between the two constituents at this interface which determines the mechanical properties of the composite as a whole. In fact, adhesion is responsible for the generally synergistic nature of composites. For example, the adhesion developed within composites can provide mechanical properties that are generally superior to the mechanical properties of the individual elements, either alone or in combination. In addition to mechanical properties, composites possessing adequate adhesional characteristics can also provide other physical properties, such as conductivity, notched impact resistance, and the like, which are superior to the sum of the properties of the individual components. A number of factors impact the adhesion developed within composites, including the dispersion of the reinforcement component within the matrix and the level of compatibility between the reinforcement and the various components which make up the matrix compositions. Coatings may be applied to the reinforcement to promote adhesion, such as the acrylate graft copolymers described in U.S. Pat. No. 4,131,577. However, there nevertheless remains in the art a need for composites exhibiting improved adhesion. There further remains a need in the art for composites having other improved propeties, such as color and the like. A variety of polymers, both thermoset and thermoplastic, commonly serve as the basis for the matrix composition. Thermoplastic polymers are particularly attractive for use in matrices, due to their ease of processability. Well known thermoplastic matrix materials include polyamides, such as nylons, polyesters, and polyolefins, particularly polypropylene. Polypropylene is a particularly attractive matrix material for applications requiring performance at low to moderate temperatures because it is relatively inexpensive and light weight, yet provides adequate physical properties. Consequently, polypropylene is regularly used as the matrix polymer in automotive composites, such as injection molded interior parts and the like.
Numerous fibrous materials are similarly known for use as reinforcements in composites. Glass fibers are particularly widely used as the reinforcing component for composites, imparting increased mechanical strength, dimensional stability, and heat resistance to the final composite. However, although glass fibers achieve desirable reinforcing properties, they are fairly costly, abrade processing equipment and increase the overall density of the composite. In certain applications, these disadvantages outweigh the advantages of using glass fibers as a reinforcement component.
Cellulosic materials have been evaluated as fibrous reinforcements for composites in the past. Klason, et al., xe2x80x9cCellulosic Fillers for Thermoplasticsxe2x80x9d, Polymer Composites, (1986); Klason, et al., xe2x80x9cThe Efficiency of Cellulosic Fillers in Common Thermoplastics. Part 1. Filling without processing aids or coupling agentsxe2x80x9d, Intern. J. Polymeric Mater., Volume 10, pgs. 159-187 (1984); Snijder, et al., xe2x80x9cPolyolefins and Engineering Plastics Reinforced with Annual Plant Fibersxe2x80x9d, The Fourth International Conference on Wood Fiber-Plastic Composites, pg. 181-191.
Cellulosic materials are especially attractive for use in composites because they have relatively low densities. For example, cellulose fibers have a density of approximately 1500 kg/m3 in comparison to a density of 2500 kg/m3 for E grade glass fibers. Such weight savings can be highly advantageous, particularly in automotive applications. In addition to the reduction in weight, cellulosic fibers are not abrasive to processing equipment in comparison to glass fibers or high density mineral fibers, e.g. wollastonite.
However, prior investigations into the use of cellulosic materials, e.g. cellulose pulps or raw lignocellulosic resources (e.g., wood flour, bagasse), in polymeric materials found that a pronounced discoloration of the composite material occurred if the cellulose materials were processed at elevated temperatures, such as the temperatures commonly employed when melt blending the reinforcement and matrix. Furthermore, cellulosic materials were found to cause significant off-gasing and objectionable odors. These disadvantageous results directed previous research efforts to the use of cellulosic materials in matrix polymers having more moderate melting temperatures, such as melting temperatures of below 200xc2x0 C. Further, the use of cellulosic fibers having higher alpha-cellulose contents has been proposed in conjunction with higher melting matrix polymers, as discussed in U.S. Pat. No. 6,270,883 hereby incorporated by reference in its entirety.
However, despite such research efforts, discoloration continues to be problematic in conventional cellulosic material-reinforced composites prepared from matrices having even moderate melting temperatures. For example, an undesirable brownish discoloration is observed in conventional composites formed from cellulose fibers dispersed in a polyolefinic matrix. As noted above, this brownish discoloration is generally associated with the degradation of the cellulose fibers during processing and often gives rise to malodors during product usage. Further, an unacceptable level of fiber agglomeration has been observed in conventional cellulosic fiber/polyolefin composites to date. As noted previously, such agglomeration would be expected to be detrimental to the interfacial adhesion characteristics of the composite, thus negatively impacting mechanical properties and the like. Further, agglomeration of the cellulosic fibers can give rise to surface roughness and non-uniform properties. Consequently, a need exists in the art for cellulose-reinforced composites having improved color and dispersion properties.
The present invention provides improved cellulosic fiber reinforced composites and methods by which to form such composites. More specifically, the present invention provides cellulosic fiber reinforced composites having a beneficial balance of matrix components selected to impart improved color and fiber dispersion characteristics to the resulting composite. The present invention further provides methods by which to produce cellulosic fiber reinforced composites having superior color and fiber dispersion in comparison to known cellulosic material reinforced composites.
The cellulosic fiber reinforced composites of the present invention further provide improved structural characteristics to the matrix material at a reduced cost in comparison to conventional glass fibers and with a much lower increase in the density of the resulting composite. The cellulosic pulp fibers employed in the composites of the invention also do not significantly abrade the processing equipment or generate malodors during composite manufacture. Additionally, the use of the cellulosic pulp materials according to the invention allows for the blending of the components and molding of the resultant composite material at lower processing temperatures.
In one advantageous embodiment, cellulose reinforced composites are provided that are prepared from a melt blending composition that includes: cellulosic pulp fibers having an alpha-cellulose purity of greater than about 80% by weight; at least one olefinic matrix polymer; at least one water soluble binder; at least one lubricant; and at least one compatibilizer. Exemplary water soluble binders include polyacrylamide, sodium carboxymethyl cellulose polyvinyl alcohol, polyethylene glycol and mixtures thereof. Exemplary lubricants for use in the present invention include silicone oil, ethylene bisstearamid, metal stearates, fatty acid amides, and mixtures thereof. In one beneficial embodiment, the lubricant is a mixture of silicone oil and ethylene bisstearamid. Exemplary compatibilizers for use in the invention include maleated polypropylene, maleated co- or ter-polymers of ethylene and mixtures thereof. Suitable olefinic matrix polymers include polypropylene, polyethylene, polybutene, polyisobutene, poly(methyl pentene), copolymers thereof, terpolymers thereof and mixtures thereof. The melt blending composition may further include at least one coupling agent selected from silanes, titanates, zirconates, and mixtures thereof.
Another aspect of the invention provides processes for the manufacture of composites containing cellulosic pulp fibers dispersed in an olefinic matrix polymer. Such processes generally include introducing a melt blending composition into the main feed throat of a compounding extruder and transporting the melt blending composition through at least three heated regions and a heated die within the extruder, in which an initial region of the compounding extruder is heated to a temperature of about 10 percent below the matrix polymer melting point; a first intermediate region of the compounding extruder, which may constitute a major region of the extruder, is heated to a temperature of about 15 to 20 percent below the matrix polymer melting point; a second intermediate region of the compounding extruder, e.g., a small region immediately preceding the die, is heated to a temperature of about 15 to 20 percent above the matrix polymer melting point; and the die is heated to a temperature of about 35 to 40 percent above the matrix polymer melting point. In further beneficial aspects, an additional amount of olefinic matrix polymer is introduced into a secondary feed throat of the compounding extruder positioned downstream of the main feed throat. In additional advantageous embodiments of these aspects, the olefinic matrix polymer introduced into the secondary feed throat possesses a melt flow index that is substantially lower than the melt flow index of the olefinic matrix polymer introduced into the main feed throat.
The present invention further provides molded articles prepared from melt blending compositions including cellulosic pulp fibers having an alpha-cellulose purity of greater than about 80 percent by weight; at least one water soluble binder; at least one lubricant; at least one compatibilizer; and at least one matrix polymer. Molded articles in accordance with the present invention include injection molded articles, compression molded articles, blow molded articles, rotational molded articles, extruded articles and pultruded articles.